許多氣體分子在中紅外光區段有吸收，所以我們可以應用這些吸收來測量各種氣體。本實驗我們測量二氧化碳氣體，雖然二氧化碳占大氣中的比例不是很高，不過它卻是一種影響氣候劇烈的氣體。二氧化碳在中紅外光的區段也有相當高的吸收，此實驗我們使用 ~2 μm 的 DFB 半導體雷射，來對二氧化碳做測量。DFB 雷射可以藉由調整注入電流或改變溫度很方便地調變輸出波長，所以是研究吸收光譜相當方便的光源。

實驗上首先我們測量二氧化碳的直接吸收，計算二氧化碳的吸收係數，探討吸收係數隨二氧化碳壓力的變化，再使用公式作曲線擬合得到Einstein A coefficient 和壓力增寬係數，這些參數可以和HITRAN的數據比較來增加實驗的可信度。在量測二氧化碳螢光的實驗中，我們克服結構設計上的問題成功地將偵測器到 light pipe 的距離拉近 2.25 mm，其螢光訊號增加到 1.68 倍；如改用偵測面積大約三倍大的偵測器，最後螢光訊號增加到約 8.7 倍。最後也使用光聲光譜的技巧測量二氧化碳，我們使用差動光聲氣室，讓有光和無光通過的共振管相減來取得光聲光譜訊號。對於直接吸收、螢光光譜和光聲光譜三種偵測方法的比較，直接吸收一次微分訊號的靈敏度約為 6.9×10-9 cm-1Hz1/2W，而螢光光譜的靈敏度可達 cm-1Hz1/2W，光聲光譜的靈敏度約為2.09×10-6 cm-1Hz1/2W，結論為吸收光譜的靈敏度約略與螢光光譜相同，而光聲光譜的靈敏度則差~30倍左右。

最後就算我們增加了螢光訊號的訊噪比，仍然還是觀察不到飽和吸收，我們將來可使用共振腔來加強雷射強度，如觀察到飽和吸收光譜，則進一步可以將雷射鎖在此飽和吸收上，建立一套2 μm 雷射的穩頻系統。

Many gas molecules have absorption in the mid-infrared region, we can take advance of this characteristics to detect molecules. In this experiment, we investigate the detection of carbon dioxide. Although the concentration of carbon dioxide is as low as 387 ppm in the air, it influences climate strongly. Absorption of carbon dioxide is very high in mid-infrared. In this experiment we use a DFB 2-μm laser diode as the light source to detect carbon dioxide. Wavelength of laser diode can be changed by changing its current or temperature. Laser diode is a very convenient light source for absorption spectroscopy.

First, we observe the absorption spectrum of the carbon dioxide, calculate the absorption coefficient of the carbon dioxide and we investigate the absorption coefficient versus pressure. We fit the experimental result using the theoretical curve to get Einstein A coefficient and pressure broadening coefficient which can be compared to the HITRAN data base. In the fluorescence experiment we reduce the distance between the detector to the light pipe by 2.25 mm. The signal is increased to 1.68 times. If we use to a new InSb detector which has about three times larger detection area, the fluorescence signal increases to 8.7 times. In addition, we have detected carbon dioxide using a differential photoacoustic cell. Finally we compare the sensitivity of absorption spectroscopy, fluorescence spectroscopy and photoacoustic spectroscopy. Sensitivity is 6.9×10-9 cm-1Hz1/2W for direct absorption, and cm-1Hz1/2W for fluorescence spectroscopy, and 2.09×10-6 cm-1Hz1/2W for photoacoustic spectroscopy. The sensitivity of absorption spectroscopy is about equal to fluorescence spectroscopy, but photoacoustic spectroscopy is smaller by 30 times.

Although we have increased the signal to noise ratio by a factor of 8.7, we still can’t observe the saturation absorption. We must setup an optical cavity to enhance the laser intensity for observing saturation absorption. Then we can lock laser on the saturation peak and build the frequency stabilization for 2-μm laser finally.

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